Readers and authors are tagging their posts with the keyword as they discuss the incident. Much of the outcry started after a publisher, Mark R. Probst, blogged about a message he received from an Amazon representative after noticing that rankings disappeared from “Transgressions” and “False Colors,” two new gay romance books.

In the message, Amazon said it excludes “adult” material from searches and best-seller lists by not calculating their sales ranks. Mr. Probst wrote that the “adult” classification didn’t apply to several of the targeted books and that some adult-oriented books, including a hardcover edition of “Playboy: The Complete Centerfolds,” continued to maintain their ranking. He also pointed to the blog of Craig Seymour, a writer who has noted his troubles with Amazon and sales rankings since February.

Amazon told Publishers Weekly that the ranking issue was due to a “glitch,” though tech blog Techdirt said that explanation resembled the one Amazon gave when negative reviews of the videogame “Spore” disappeared from the site in September.

By Sunday evening, “AmazonFail” tweets were flooding the microblogging site. The Meta Writer blog began compiling a list of authors whose works were de-ranked. An online petition quickly drew thousands of signatures. Some book bloggers penned open letters to the company, while others suggested a Google-bombing campaign to promote a sarcastic definition of “Amazon rank.”

UPDATE: Neil Giuliano, president of the Gay & Lesbian Alliance Against Defamation, says in a statement: “GLAAD has reached out to Amazon.com and they indicate this was an error, so we expect to start seeing evidence of its correction immediately, and any loss of visibility of gay-themed books as a result of this error will be made right by Amazon … When people learn about the lives of gay and transgender people and the common ground we share, the culture changes and advances. It is so important that stories about the lives of our community are available, and that companies like Amazon promote these titles in an equal fashion.”